Tag: Silicon

How to Build a Computer 39: Epitaxy II: The Reckoning

 

I know y’all have been waiting eagerly with your wafer in the chamber, the temperature pumped down and your native oxide layer stripped off for me to finish this two-parter. Well, wait no longer! Okay, maybe wait some as you have to find a tank of dichlorosilane to hook up so you have something to epitax onto your wafer. Di-what now?

Dichlorosilane! Or Tri- or Tetra; really anything from SiH4 to SiCl4 works, though I’m told industry generally works with SiCl2H2. Alright, you pump in dichlorosilane gas and react it on the wafer and it puts silicon on top of your silicon. Neat, huh? That’s it! Join us fortnight next for —

How to Build a Computer 37: CVD II, This Time It’s Personal

 

Last time, if you’ll recall, I discussed the basic idea of a chemical vapor deposition system, and described how you’d use it to deposit silicon onto your wafer. Today we’re following rather directly from that post, where we answer some important questions. Questions like “What if I don’t want to put down silicon? What other things can you offer me?” Well, for starters

SiCl4(g) + 2H2(g) + O2(g) —> SiO2(s) + 4HCl(g) ~900 C

How to Build a Computer 36: Chemical Vapor Deposition

 

No matter how much fun you’re having etching silicon, applying and stripping photoresist, or implanting ions, sooner or later you’re going to have to actually put down some lines. Gotta build a circuit eventually. Chemical Vapor Deposition (CVD) is one of the main ways this gets done. Let’s have a look at what we’re doing, shall we?

If I had known I was going to use this picture at least three times I might have put a little more effort into the sketching.

How to Build a Computer 35: Anisotropic Etching

 

Last time we talked about how to make tiny little holes in silicon using harsh acids. Wet etching is fine and all, but sometimes you just can’t make a feature small enough. You’re limited by the aspect ratio. That is, how wide it is versus how tall it is. A post hole has a high aspect ratio because it’s much deeper than it is wide. A strip mine is a pretty low aspect ratio hole. The difficulty with making high aspect ratio holes in your silicon is that your etchant is going to etch down, yes, but it’s also going to etch towards the sides.

Before we get into dry etching there’s one more trick for making an anisotropic (uh, it etches downward quicker than it goes sideways. Literally the word means not-the-same-in-all-directions.) wet etch. What happens if you do your etching with a strong base instead of a strong acid? As it turns out, and for no reason, I’ve managed to determine, a strong base will etch one crystal face preferentially.

How to Build a Computer 34: Etching

 

We’re moving back from the series on measurement to the whole process of making computer bits out of silicon. Way back, starting with Computers 7, I started a series on patterning; how you can take an idea and draw it small enough that you can apply that pattern to these really tiny circuits. I went over, step by step, each thing you need to do to create the pattern. I skipped entirely the bit where I tell you what, exactly, you do with one of those patterns when you’ve got them. This is the first of a couple of articles that fit, in manufacturing terms, between Computers 15 (Developing), and 16 (Stripping). You develop your pattern on with photoresist, this is how you make it permanent.

We’ll start with etching. Broadly speaking ‘etching’ covers any process where you start with more material and end up with less material. I mean aside from gambling. Let’s say you’ve got your silicon wafer, you want to etch some of that silicon away. To do this we start by burning your wafer. …Okay, perhaps that’s poor phrasing. Put the flamethrower down and I’ll describe what I mean. To protect your silicon wafer from the damage the etching process would do to it we’re going to want to mask it, with a silicon dioxide layer. Heat your wafer up in the presence of oxygen and this happens:

How to Build a Computer 4: Diodes and Transistors

 

We all love blasting things with ions, and most of us could spend all day shooting at wafers, but eventually someone is going to ask you to build something useful. What am I doing with all this mess of silicon anyhow? Here’s where we see the use of all that stuff. What do you suppose happens when you put a p-doped chunk of silicon next to an n-doped chunk of silicon?

One last cookie photo, then I’m going on a diet. Swearsies.

How to Build a Computer Part 2 of N: Crystallography

 

Last week we saw how to turn sand into silicon. This week I was planning on showing you how to turn silicon into a semiconductor. I mean more of one than it already is. Unfortunately my brief notes on crystallography went long. This week we’ll discuss crystals, next week we’ll do doping, and the week after that we’ll finally get to transistors. Unless I wax even more loquacious, which is the way the smart money is betting.

In a crystal every atom is slotted neatly into an ordered lattice, and every spot in the lattice has an atom in it. With some exceptions. Actually those exceptions are most of what we’re going to talk about today. Let’s assume this is a perfect silicon crystal:

How to Build a Computer, Part 1 of N: Silicon

 

As the illustrious @JohnWalker no longer treads these halls, I figured there was an opportunity to thrust my metaphorical booties into his clodhoppers. I’ve been kicking the idea of this series around for a long time. Broadly speaking it covers everything you need to know to build a computer. Everything. Today, we’re going to learn how to make silicon wafers.

He’s Gone Silicon